High viscometric index, low volatilities, superior low temperature as well as thermal/oxidative stabilities are those of the most desirable and essential performance characteristics exhibited by synthetic lubricant base stocks such as polymerized alpha olefin base oils known/abbreviated as PAO. For instance, U.S. Pat. No. 6,869,917 teaches the use of PAO in the formulation of synthetic engine oil wherein PAO was claimed to exhibit viscometric index of, preferably, greater than 120, Selby-NOACK (abbreviated as NOACK) volatility from 4 to about 12%. A minimum NOACK volatility specification of 13% will be required for the future heavy diesel engine requirements. The value 13% NOACK is the diesel lubricant specification proposed by Cummins in 2007, and cited as the minimum volatility for diesel engine oil other than 10W grades, by ACEA, EMA, and JAMA associations, Global Performance Specification for light duty diesel engine oil. Additionally, PAO was recognized as a wax free, synthetic composition with superior low temperature properties which are important for engine oil applications as it enhances engine oil pumpability/start up performance especially in cold climate conditions, and for a number of lubricant and additive applications that has no tolerance of crystallization/wax formation at low temperatures. Refrigeration oil is an example that requires wax-free and outstanding low temperature properties, described in detailed in the Research Report sponsored by Air-Conditioning Research Institute, titled “Using Acid Number as indicator for refrigeration system performance”, ARTI-21CR/611-500, Final Report, 1993. Substantially linear alkylphenols are proposed to ensure the wax free operations, see U.S. Pat. No. 5,600,025. For instance, the minute presence of waxy components in refrigeration oils is known to cause the capillary blockage and thus system failures. Waxy constitute in lubricants and additives can plug the in-line filtering devices in additive distribution systems and the fuel or lube systems of actual operating engines. Such a plugging would obviously be catastrophic and must be avoided.
Viscometric index higher than 120 is also the critical performance specification that separates GIII base oil, one of the highest categories of mineral oil based products, from that of lower grade GI and GII base oils. Because of this improvement and other performance benefits that are comparable to PAO with the exception in the region of low temperature properties, GIII base oil today is considered as “synthetic” base stock in Europe and North America.
Despite these good performance characteristics, the above mentioned synthetics including PAO and GIII base oils, have a poor solvency toward additive and hence often require a second base oil component such as ester or other oxygen containing base oils such as oil-soluble PAG or polar base stock such as carbonates. See “Esters in Synthetic Lubricants and High Performance Functional Fluids” (2nd. ed.), Rudnick L R and Shubkin R L (eds.), Marcel Dekker, New York, pp. 63-101. Esters have good additive solvency, however, is prone to hydrolysis and thermal/oxidative breakdown resulting in high acid/sludge formation tendency. Oil soluble PAG or other polar base stocks offer some performance benefits, however, are often hygroscopic and problematic with compatibility with system components and the rest of the lubricant itself. Accordingly it is highly desirable to have a synthetic fluid that shall meet all of the above performance requirements and in the meantime to provide the additional performance features such as built-in solvency toward additives.
Synthetic heavy alkyl benzene fluids, hereinafter referred as “heavy alkylates”, are commercially known as the by-products isolated from the detergent alkylate production by reacting olefins or paraffins feedstock with benzene. See the Handbook of Petroleum Refining Processes, McGraw-Hill, New York, Second Edition, 1977, by R. A. Meyers, pages 153-166. These heavy alkylates provide built-in solvency, excellent low temperature properties, and improved thermal/oxidative stability when compare with petroleum derived mineral oils, however, exhibit generally poor viscometric index and high volatility due to the fact that they contain a significant portion of isomerized/rearranged products other than the re-alkylated product of the detergent alkylate that include dialkylbenzene (abbreviated as DAB) as well as higher alkylates. For instance, heavy alkylates based on branched olefins such as propylene tetramers gave poor VI less than 30 to even negative values, while heavy alkylates based on linear olefins gave viscometric index generally between 40 and 110 that is not any better than those of the traditional mineral oil based products. Because of these deficiencies the use of heavy alkylates are used as the synthetic substitutes for naphthenic oils and are limited to applications where it does not place high demand on volatilities and viscometric index such as those in hermetically sealed systems, e.g., refrigeration oil, heat transfer and insulating fluid applications.
U.S. Pat. No. 3,173,965 discloses the high yield synthesis of DAB with a minimum of viscometric index of no less than 95 and a pour point of no more than 15° F. (−9.4° C.) through a two-step alkylation process in the presence of Friedel Crafts liquid acid catalysts such as HF, AlCl3, BF3, etc. Suitable alkylating agents include pure linear alpha olefins or olefins made by wax or petrolatum cracking processes.
U.S. Pat. No. 4,148,834 further teaches the use of AlCl3 or AlBr3 in the above second step alkylation process wherein it improves the viscometric index of products that are made with HF catalyst.
U.S. Pat. No. 3,288,716 discloses the use of by-product alkylate bottom with a minimum viscosity index of at least 90, a −40° C. viscosity of no higher than 25,000 cSt. and a pour point of at least—70° C., and a flash point of at least 430° F.
U.S. Pat. No. 4,011,166 describes the simultaneous polymerization-alkylation process by reacting shorter chain linear alpha olefins with benzene. Such chemistry gives alkylate products with much higher viscosity and somewhat improved viscometric index but not the low temperature properties wherein patentee points out the difficulties in improving pour points when carbon number is C14 and above.
U.S. Pat. No. 6,491,809 teaches the production of DAB through the use of shape-selective solid catalysts by reacting benzene with linear alpha olefins. The shape-selectivity of the catalysts minimizes the alkyl side chain isomerization and enables the exclusive formation of DAB with exceptionally high 2-linear isomer content. The formation of high 2-linear isomer content gives the high viscometric index; however, it results in a DAB product with exceptionally high pour points, poor cold cranking viscosity, as well as high crystallization tendency at temperature as high as 0° C.
The present invention discloses the use of olefins with slight/controlled degree of branching, hereinafter called Slightly Branched Olefins, in the production of the DAB composition with a surprising and novel combination of performance benefits unmatched by the heavy alkylates as well as those described in the prior arts of the above. This combination of performance benefits qualifies the DAB composition of the present invention as a stand-alone and premium synthetic base stock for a broad range of lubricant applications. It's built-in benzene moiety provides the inherent solvency toward additives as well as additional performance benefits such as improved lubricity with and without additives, and hence it can also be used as ester replacements as a second base oil or additive component.